thunderfvck - 23-2-2004 at 16:26
Hello.
We recently did a lab synthesizing tropylium fluoborate. We added acetic anhydride to a flask, colled it in an ice bath and then added some fluoboric
acid. Upon addition of the fluoboric acid, I heard hissing sounds as the acid hit the acetic anhydride, what exactly was going on? What makes this
reaction so exothermic, what are the products? Once it was all added we added some triphenylmethanol which created a beautiful dark orange color as I
recall. Then we added some 1,3,5-cycloheptatriene until the color faded to a pale yellow. We used ether to drag the crystals out of solution.
Tropylium fluoborate is basically 1,3,5-cycloheptatriene with a hydrogen missing on the sp3 carbon, the positive charge is delocalized around the
ring. Yes, we isolated a carbocation as a salt! It was quite exciting.
Okay. We were also given the IR and NMR spectra of 1,3,5-cycloheptriene and tropylium fluoborate. This is where most of my problems lie. Well not so
many but anyway.
http://www.aist.go.jp/RIODB/SDBS/sdbs/owa/sdbs_sea.cre_frame...
Here is a link to the IR spectra I recieved (curtosity of SDBS), scroll to IR in KBr disc.
The broad portion from 3200-3600, what causes this? I learned that mostly broad portions are caused by H-bonding in solution. Is this the case? Could
the positive charge (which is delocalized around the ring, it's aromatic) be creating H-bonding among fellow molecules, or to some effect similar
to this? And...Well that's about it.
I also have a question about the ether being used to drag the crystals out of solution. Why use ether (diethyl ether of course)? Ether is quite non
polar, yes, and the salt is fairly polar (I'm not sure about this, please clarify, but I'm going on the assumption that since it's a
carbocation it's polar, seems to make sense). So is it because of these differences that the salt is pulled out of solution? Why not use water,
because it is much too polar and would simply dissolve the salt?
AND I've got a question about the use of acetic anhydride in this reaction. I've read that the reaction is pushed in the forward direction
because of the acetic anhydride. Because when triphenylmethanol reacts with fluoboric acid, water is released, this water decomposes acetic anhydride
to acetic acid. In order for the carbocation (the newly formed trityl fluoborate --> the triphenylmethanol without it's OH group and now a +
on the carbon) to survive it must be present in an acidic environment. So the acetic acid promotes the development of more product this way, and it
also eats the water that had been removed from triphenylmethanol and so the system tries to produce more (le chatelier) product. This is my
understanding of the use of acetic anhydride, please tell me where, if I did, go wrong. One thing I don't understand is that hissing sound I had
heard earlier, I can't see what could have happened as a reaction if the real use of acetic anhydride only comes in when the triphenylmethanol is
present. So why cool it, what's going on in there?
Okay, this was quite long, and I realize I've asked quite a few questions all of which are more or less spread apart from each other. Anyway,
thanks a bundle.
[Edited on 24-2-2004 by thunderfvck]
[Edited on 24-2-2004 by thunderfvck]
[Edited on 24-2-2004 by thunderfvck]
guaguanco - 23-2-2004 at 22:01
Nice experiment!
Ether is commonly used to force salts out of solution from polar organic solvents, because of it's non-polarity. Water would be about the worst
possible thing to add if you wanted to isolate crystals because of it's extreme polarity.
The acetic anhydride does gobble up the water, and I would expect there's a sizeable excess in your reaction. Your description of it's role
seems correct.
One could hypothesize a complex:
CH3COOCOCH3 + HBF4 -> CH3COOH + CH3CO(+)BF4(-)
Another possibility: the fluoroboric acid is protonating the carbonyls to form a complex like
CH3COOC(CH3)=O(+)H BF4(-)
(Imagine an acetic anydride molecule with a proton on one of the carbonyls).
HBF4 is such an intense Lewis Acid that it protonates almost anything in it's path; either of these pathways could well be exothermic (hissing).
These guesses are worth what they cost you...
[Edited on 24-2-2004 by guaguanco]
unionised - 25-2-2004 at 14:08
I think that IR absorbtion might be water. KBr is hygroscopic.